Sewing machines having a sewing area, a feeder assembly to advance material being sewn through the sewing area in a stepwise manner and a drive crank pivotable to-and-fro to move the feeder assembly intermittently in the direction in which the material being sewn is advanced are known in the art. In such arrangements, the drive crank is pivotably supported at one end and is pivoted to-and-fro by a rod connected to another end of the drive crank and driven by a cam of a drive shaft. An output crank projects radially from an output shaft to which it is affixed, and the two ends of a linkbar are connected to the other end of the drive crank and to the free end of the output crank, respectively. The output shaft comprises a radial arm having a free end linked to one end of a support beam bearing the feeder assembly for the material being sewn. At its other end, the support beam cooperates with a second cam revolving synchronously with the first cam for the purpose of intermittently raising the feeder assembly in time with its intermittent motion in the advance direction of the material being sewn.
In these known sewing machines, to adjust the length of each movement of the feeder assembly in the direction in which the material being sewn is advanced and to adjust thereby the stitch-length of the sewing machine, the pivot axis of the drive crank is displaced. When this pivot axis is aligned with the axis about which the linkbar and the output crank are mutually pivotable and when the drive shaft is running, the drive crank and the linkbar will pivot about the same axis and the linkbar will not displace the output crank whereby the output shaft is at rest. This corresponds to a stitch-length of zero. As the pivot axis of the drive crank is displaced from this position away from the drive shaft toward the output shaft forming the pivot axis of the output crank, the stitch-length is increased. The pivot axis of the drive crank is displaced by means of a stitch-length adjustment device comprising an adjustment shaft extending parallel to the drive shaft and to the output shaft and fitted with a radial arm supporting a bolt which forms the pivot axis of the drive crank. This pivot axis extends parallel to the adjustment shaft.
A known stitch-length adjustment device comprises a manually rotatable adjustment cam cooperating with the adjustment shaft by means of a rod linkage. For purposes of seam locking, the rod linkage can be actuated by a manually pivotable lever independently of the adjustment cam to rotate the adjustment shaft against a spring-loading so that the pivot axis of the drive crank moves through the zero stitch-length position away from the output shaft, as a result of which the feeder assembly when raised always moves opposite the normal direction of advance of the material being sewn. It is also possible to drive in this manner upper and lower auxiliary feeder assemblies each provided with an adjustment shaft coupled by a rod linkage to the adjustment shaft of the main feeder assembly. These two rod linkages are adjustable to assure a specific differential advancement of the material being sewn (German Offenlegungsschrift 26 25 152).
Another known stitch-length adjustment device comprises a double-acting compressed-air cylinder to actuate the adjustment shaft in either direction, the cylinder performing equal strokes in both directions which are adjustable in accordance with the particular desired stitch-length by manually rotating an adjustment sleeve on the cylinder housing. The piston rod of the compressed-air cylinder is linked to a radial arm of the adjustment shaft (German patent 29 16 642).
Further, a stitch-length adjustment device is known which comprises a digitally operating compressed-air cylinder driving the adjustment shaft and a control system for the compressed-air cylinder connected to an operating panel for setting a desired stitch length in the control system. Lower and possibly upper auxiliary feeder assemblies, may be provided to allow a differential advancing of the material being sewn, the lower auxiliary feeder assembly or both auxiliary feeder assemblies and the main feeder assembly being driven identically. An additional adjustment shaft for the single or both auxiliary feeder assemblies is provided which also is adjusted by a digitally operating compressed-air cylinder controlled in the same manner and acting on a radial arm of the adjustment shaft. In this design, a purely mechanical stitch-length adjustment device instead of an electro-pneumatic stitch-length adjustment device may be provided for the main feeder assembly (German Offenlegungsschrift 31 18 964).
It is also known with respect to sewing machines of the type indicated to take special steps to achieve specific functions, namely to sew a seam accurately to a predetermined location (German patent 31 50 141; German Offenlegungsschrift 33 24 715 and German patent 33 42 391) or to stitch together two same-pattern cloth layers so as to match the patterns (German patent 33 46 163). An electric stepping motor may be used therein (German Offenlegungsschrift 33 24 715) even for rotating the adjustment cam (German patent 33 46 163).
Moreover a stitch-length adjustment device with an electric stepping motor is provided in a further known sewing machine differing slightly from the above cited sewing machines in that the drive crank is pivotably supported on one arm of an angle-lever and the other arm of which is acted on through a rod by the electric stepping motor in order to pivot the angle-lever to adjust the drive crank pivot axis. The output crank is rotatably supported on its pivot axis and is provided with a hub from which an arm radially projects that encloses, with a fork-shaped free end, a pin at the adjacent end of a crossbeam of the feeder assembly advancing the material being sewn in order to displace this crossbeam end on a guide pin extending in the direction of advance of the material being sewn. The crossbeam rests on the other end near the feeder assembly by means of a lower projection on the cam for raising the feeder assembly. This type of sewing machine does not comprise an adjustment shaft nor an output shaft (German Offenlegungsschrift 33 21 215).